This invention relates to a radio receiver, and more specifically, to a radio receiver having a receiving frequency setting mode for controlling the output frequency of a local oscillator by means of a phase locked loop (hereinafter referred to as PLL), and a tuning holding mode for maintaining the output frequency of the local oscillator under the control of an automatic frequency control (hereinafter referred to as AFC) loop.
Generally known are various radio receivers which have a receiving frequency setting mode for setting the output frequency of a voltage-controlled oscillator (hereinafter referred to as VCO) serving as a local oscillator to a desired frequency by means of a PLL which includes a phase comparator for comparing the phase of a signal corresponding to a local oscillator signal produced by the VCO with the phase of a reference signal with a given frequency, and a tuning holding mode for maintaining the output frequency of the VCO by means of an AFC loop which includes a tuning detector circuit to produce an S-curve signal representing the tuning condition of the receiver and negatively feeds back the S-curve signal to the VCO.
In a copending U.S. patent application Ser. No. 195,474 filed on Oct. 9, 1980 and assigned to the same assignee as this invention, there is disclosed a radio receiver in which the PLL and the AFC loop have a common loop filter for supplying the VCO with a control voltage. Such radio receiver is advantageous in cost since it requires only a single loop filter for two control loops.
Practically, however, the transfer function required of the loop filter varies with operation modes, that is, receiving frequency setting mode and tuning holding mode. When using the common loop filter, therefore, the efficiency of operation in one of those two modes need inevitably be sacrificed.
Namely, in the receiving frequency setting mode, it is desired that the time required for the actual setting of a receiving frequency after designation of the receiving frequency should be minimized. Accordingly, the time constant of the loop filter should be as small as possible. In the tuning holding mode, on the other hand, the stability of tuning is strongly desired, so that the time constant of the loop filter need be greater than the time constant used in the receiving frequency setting mode.